The International Telegraph and Telephone Consultative Committee (CCITT) has established a set of recommendations describing the methodology for a recently-proposed digital transport network called Synchronous Digital Hierarchy (SDH).
These recommendations cover the transport frame structure, multiplexing methods, basic outlines of the equipment functionality and the means of managing this equipment. The directly relevant recommendations are: G.707, G.708, G.709, G781, G.782, G.783, G.784, G.773, G.sdx1, G.sdx2, G.sdx3, G.sna1, G.sna2, G.81s, and G.82j. Furthermore, the recommendations are backward compatible with the existing PDH recommendations of G.702, and G.703, etc.
The CCITT recommendations are concerned with functionality and are not specific to particular equipment implementation strategies. Therefore it is possible to combine several specific functional blocks to form a particular equipment type.
The present invention can provide equipment conforming to the relevant standards set out in the recommendations, and can assist in arriving at integrated system solutions to equipment design problems in particular SDH applications.
In order to allow items of communications equipment on different networks such as two optical ring networks to communicate with one another, conventionally a Digital Cross Connect element (DXC) is provided. This DXC is essentially a digital switching matrix with an operation interface for setting up relatively static connection between input and output signals or channels.
In such a conventional cross connect, as defined by the CCITT, all the interconnecting traffic from the ring networks needs to pass through the cross connect switch, and the data stream of each ring is demultiplexed into its constituent channels and these channels are then all applied to the switching matrix. Thus, even those channels of one ring that are not required to be routed through to the other ring pass through the matrix, and this gives rise to a number of problems as discussed briefly below.
Firstly, the switching capacity of the switching matrix must be large since all of the ring channels must pass through it. Commonly only a few through channels between two rings are actually required, so that the conventional DXC as outlined above is wasteful of hardware and hence expensive to install. In addition, a major problem with rings of all kinds is that if the service demand grows unexpectedly on a portion of the ring the whole ring must be re-engineered and new capacity installed. Since the conventional DXC is already expensive and inherently wasteful of capacity it is not normal to provide spare capacity, so that as the traffic on the ring increases the DXC requires expensive modification or even complete replacement.
The other problem is interconnection of several ring networks together while keeping the integrity of each ring unaffected. In this respect in the conventional DXC the constituent channels (virtual containers--VCs) are effectively terminated in the cross connect and the path overhead of the constituent VC traffic passing through the DXC must be regenerated. This presents a problem in preserving the path continuity and path monitoring from end to end as is desired in all SDH networks.